Achieve timing closure: DSP48E1 pipelines, 4-stage NCO, 28-bit CIC, ASYNC_REG

Phase 0+ timing optimization (attempts #13-22 + implementation):

NCO (nco_400m_enhanced.v):
- 4-stage pipeline: DSP48E1 accumulate -> LUT read -> negate -> quadrant MUX
- DSP48E1 phase accumulator in P=P+C mode (eliminates 8-stage CARRY4 chain)
- Registered phase_inc_dithered to break cascaded 32-bit add path

DDC (ddc_400m.v):
- Direct DSP48E1 instantiation for I/Q mixers (AREG=1, BREG=1, MREG=1, PREG=1)
- CEP=1, RSTP=!reset_n for proper pipeline control
- 3-stage dsp_valid_pipe for PREG=1 latency
- Behavioral sim model under ifdef SIMULATION for Icarus compatibility

CIC (cic_decimator_4x_enhanced.v):
- 28-bit accumulators (was 36) per CIC width formula: 18 + 5*log2(4) = 28
- Removed integrator/comb saturation (CIC uses wrapping arithmetic by design)
- Pipelined output saturation comparison

CDC/ASYNC_REG:
- ASYNC_REG attribute on all CDC synchronizer registers (cdc_modules.v,
  radar_system_top.v, usb_data_interface.v)
- Sync reset in generate blocks (cdc_modules.v)

Results: Vivado post-implementation WNS=+1.196ns, 0 failing endpoints,
850 LUTs (1.34%), 466 FFs (0.37%), 2 DSP48E1 (0.83%) on xc7a100t.
All testbenches pass: 241/244 (3 known stub failures).

9_Firmware/9_2_FPGA/tb/nco_quadrature.csv

@@ -7,16 +7,16 @@ sample,sin,cos,mag_sq
 5,31113,9512,1058496913
 6,32285,4808,1065438089
 7,32757,0,1073021049
-8,4808,-32285,1065438089
-9,9512,-31113,1058496913
-10,14732,-28898,1052126228
-11,18868,-26319,1048691185
-12,22594,-23170,1047337736
-13,26319,-18868,1048691185
-14,28898,-14732,1052126228
-15,31113,-9512,1058496913
-16,32285,-4808,1065438089
-17,32757,0,1073021049
+8,32285,-4808,1065438089
+9,31113,-9512,1058496913
+10,28898,-14732,1052126228
+11,26319,-18868,1048691185
+12,23170,-22594,1047337736
+13,18868,-26319,1048691185
+14,14732,-28898,1052126228
+15,9512,-31113,1058496913
+16,4808,-32285,1065438089
+17,0,-32757,1073021049
 18,-32285,-4808,1065438089
 19,-31113,-9512,1058496913
 20,-28898,-14732,1052126228
@@ -27,15 +27,15 @@ sample,sin,cos,mag_sq
 25,-9512,-31113,1058496913
 26,-4808,-32285,1065438089
 27,0,-32757,1073021049
-28,-32285,4808,1065438089
-29,-31113,9512,1058496913
-30,-28898,14732,1052126228
-31,-26319,18868,1048691185
-32,-23170,22594,1047337736
-33,-18868,26319,1048691185
-34,-14732,28898,1052126228
-35,-9512,31113,1058496913
-36,-4808,32285,1065438089
-37,0,32757,1073021049
+28,-4808,32285,1065438089
+29,-9512,31113,1058496913
+30,-14732,28898,1052126228
+31,-18868,26319,1048691185
+32,-22594,23170,1047337736
+33,-26319,18868,1048691185
+34,-28898,14732,1052126228
+35,-31113,9512,1058496913
+36,-32285,4808,1065438089
+37,-32757,0,1073021049
 38,4808,32285,1065438089
 39,9512,31113,1058496913